Bleaching Of Chemical Pulps Research Articles

Synergism of enzymes in chemical pulp bleaching from an industrial point of view: A critical review

AbstractEnzymes are biological catalysts and are very specific, catalyzing either a single chemical reaction or a limited number of closely related reactions. For example, xylanases are enzymes that catalyze the cleavage of polymeric xylan and thereby break down this hemicellulose. The first xylanase enzyme preparations used in the bleaching process of chemical pulp also contained cellulase, which catalyzes the hydrolysis of cellulose. This obviously had an adverse effect on pulp yield and quality. Unfortunately, this setback gave enzyme‐assisted pulp bleaching a negative reputation. At a later stage, enzyme producers managed to engineer enzyme production strains that generated cellulase‐free xylanase preparations. However, due to the initial negative experiences with the earlier enzyme mixtures, only a limited number of companies in the pulp industry have seriously considered using these so‐called second‐generation enzymes in their bleach plants. It is apparent that these improved enzyme preparations would bring about significant benefits in terms of chemical cost savings and effluent quality. In addition to xylanase‐aided bleaching, it is possible to improve the effectivity further by adding other enzymes, such as lipase and esterase, to create an enzyme cocktail. This may be particularly beneficial in the bleaching of hardwood pulp, such as white birch, which often encounters complex and troublesome problems with wood extractives. By adding different types of enzymes at more than one position in the fiberline, even further improvements are possible. The main objective of this review is to discuss the advantages of incorporating modern enzyme preparations in the bleaching of chemical pulp.

Detailed modeling of the kraft pulping chemistry: carbohydrate reactions

AbstractThe article introduces a detailed model for carbohydrate chemistry in kraft pulping. This article is continuation to the modeling work carried out for hot water extraction and chemical pulp bleaching. The model includes galactoglucomannan, xylan, and cellulose acid–base equilibria, in addition to peeling, stopping, and alkaline hydrolysis reactions of the same carbohydrates, as well as hexenuronic acid formation and degradation reactions. The Arrhenius parameters were applied from the literature or regressed against experimental data in the present study. The model is very successful in predicting the experimental data of carbohydrate reactions during kraft pulping. Many features of the pulping‐related model can be applied to specific fractionation chemistry considerations. The detailed knowledge on carbohydrates composition at any stage of pulping gives possibility for further development of biorefinery cases based on kraft pulping, such as biofuel and chemicals production.

Rapid and Selective Catalytic Oxidation of Hexenuronic Acid and Lignin in Cellulosic Fibers

Processes called pulping and bleaching are required for preparing industrial scale pure cellulose from biomass. Bleaching generally purifies the cellulose from components called hexenuronic acid (HexA) and lignin. Nevertheless, the oxidation reactions of these bleachable components generally consume stoichiometric volumes of oxidant and hours of retention time. The efficiency of the oxidants can be enhanced by catalysis. The present study focuses on increasing the selectivity of hypochlorous acid (HOCl) produced during chlorine dioxide bleaching toward oxidation of HexA and lignin with the help of a tertiary amine as a catalyst. A strong electrophile forms when HOCl reacts with tertiary amine, which further reacts selectively and efficiently with HexA and lignin. Catalytic oxidation experiments were conducted at broad range of pH, temperature, and oxidant dosage in conditions that are milder than used today in industry. Surprisingly, catalytic amounts of industrial tertiary amine were used, and the oxidation reactions took place in seconds. This study opens the door for future chemical pulp bleaching technologies with reduced oxidation time, less chemical consumption, and high chemical oxidation efficiency.

Optimization of Xylanase Production fromAspergillus nigerfor Biobleaching of Eucalyptus Pulp

A crude endo-xylanase produced by Aspergillus niger BCC14405 was investigated for its potential in pre-bleaching of chemical pulp from eucalyptus. The optimal fermentation conditions on the basis of optimization using response surface methodology included cultivation in a complex medium comprising wheat bran, rice bran, and soybean meal supplemented with yeast extract, glucose, peptone, and lactose with a starting pH of 6.0 for 7 d. This resulted in production of 89.5 IU/mL of xylanase with minor cellulase activity. Proteomic analysis using LC/MS/MS revealed that the crude enzyme was a composite of hemicellulolytic enzymes, including endo-β-1,4-xylanase and other hemicellulolytic enzymes attacking arabinoxylan and mannan. Pretreatment of the pulp at a xylanase dosage of 10 IU/g increased the brightness ceiling after the C-Eop-H bleaching step up to 3.0% using a chlorine charge with a C-factor of 0.16-0.20. Xylanase treatment also led to reduction in chlorine charge of at least 20%, with an acceptable brightness level. The enzyme pretreatment resulted in a slight increase in pulp viscosity, suggesting an increase in relative cellulose content. The crude enzyme was potent in the enzyme-aided bleaching of chemical pulp in an environmentally friendly pulping process.

Polyoxometalate Delignification of Birch Kraft Pulp and Effect on Residual Lignin

To advance the understanding of delignification with polyoxometalates (POMs) that have been explored for use in bleaching of chemical pulps, the transformation of lignin during anaerobic treatment of birch kraft pulp with an equilibrated POM mixture composed of Na5(+2)[SiV1(-0.1)MoW10(+0.1)O40] was investigated. The conversion factor between the Klason lignin and the kappa number corrected for the hexenuronic acid (HexA) contribution gradually increased, indicating loss of lignin oxidizability. Comparative analysis of residual lignins isolated from pulps of decreasing kappa number showed that lignin undergoes changes that include a sharp reduction in the content of PhOH groups, a gradual demethylation, and a high increase in carbonyl groups. The results indicated that the POM treatment of kraft pulps leads to the loss of aromaticity, most likely caused by the conversion of aromatic rings to quinone moieties. The 2D NMR studies revealed the disappearance of the correlations assigned to stilbene structures formed during kraft pulping, and the weakening of those assigned to the native lignin bonds. The GPC studies showed a gradual lignin depolymerization.

Inclusion of a pressurized acidolysis stage in chemical pulp bleaching

Hardwood soda-AQ pulps are believed to be rich in benzyl sugar ethers (BSE) that can be partially cleaved by aqueous acidic treatments. The aim of this investigation was to evaluate the effect of acidolysis on final bleached brightness for kraft and soda-AQ (SAQ) hardwood pulps. The increase in final brightness due to acidolysis at 110 °C was twice as high for a eucalyptus SAQ pulp as compared to the kraft pulp. An oxygen delignified maple C-SAQ pulp (carbonate pre-treated SAQ) was acidolyzed at 120 °C and pH 2.6 for 30 min. When 1.60% ClO2 + 0.25% H2O2 on pulp was used in DEPD final bleaching of the control sample a brightness of 91.5% was achieved. When only 1.00% ClO2 + 0.25% H2O2 on pulp was used for the acidolyzed sample a brightness of 92.0% was attained. Analyses of the maple pulp after the acidolysis showed no major change in lignin content, brightness, or pulp yield. The minor changes suggest that a facile reaction such as benzyl ether cleavage was responsible for the improved bleachability. Preliminary research involving a lignin model compound and commercial birch xylan showed that lignin-carbohydrate condensation products were generated under SAQ cooking conditions. Furthermore, a fraction of these lignin-carbohydrate moieties were subsequently cleaved by acidolysis at pH 2.5 and 105 °C.

The final bleaching of eucalypt kraft pulps with hydrogen peroxide: relationship with industrial ECF bleaching history and cellulose degradation

AbstractBACKGROUND: The process of chemical pulp bleaching is based for the most part in chlorine dioxide within elemental chlorine free (ECF) technologies. The use of greener alternatives such as bleaching with hydrogen peroxide (P stage) is not widely used owing to selectivity concerns related to transition metal‐catalyzed decomposition reactions. Even at the final stage where peroxide is recognized to boost brightness and improve the brightness stability of the bleached pulp, cellulose degradation often overcomes these advantages. This paper presents the results of studies intended to optimize final peroxide bleaching performance considering two standard ECF industrial bleaching sequences: the conventional DED and the ECF‐light OQ(PO)D (stages name: D—chlorine dioxide; E—alkaline extraction; O—oxygen; Q—chelation, (PO)—hydrogen peroxide pressurized with oxygen).RESULTS: The addition of sodium diethylenetriaminepentaacetate (DTPA) was the most effective option in terms of DED pulp bleachability and selectivity with hydrogen peroxide, as well as in terms of brightness reversion. As regards the OQ(PO)D pulp, a blend of DTPA and magnesium was the most beneficial in those properties.CONCLUSIONS: The choice of the best hydrogen peroxide stabilizer, among the different tested combinations of magnesium and chelants (EDTA and DTPA) studied, in terms of pulp bleachability, bleaching selectivity and brightness reversion is dependent on the impact of the previous bleaching stages on metallic nature and content. The pulp Mg/(Fe + Cu) ratio was highlighted as a process parameter controlling cellulose degradation in peroxide bleaching. Copyright © 2010 Society of Chemical Industry

ENVIRONMENTALLY FRIENDLY TECHNIQUES FOR CHEMICAL PULP BLEACHING

In the last decades there has been a rapid evolution of techniques for production of bleached pulp. Much of these processes have been environmentally driven. New techniques have been developed in order to replace chlorine-based reagents in producing bleached pulp. Reducing the environmental impact of pulp production can be attain by using non-pollutant bleaching reagents like oxygen and related compounds. This paper deals with present findings and general trends in environmentally-sound chemical pulp bleaching. Most used environmentally-friendly reactants for pulp bleaching are presented emphasizing their role in a bleaching sequence. Oxygen, hydrogen peroxide, ozone, peracids and polyoxometalates are identified as among the most important reagents that can bleach pulp without affecting the environment. Environmentally-friendly pulp bleaching techniques which include a combination of oxygen, ozone, hydrogen peroxide and other non-chlorine chemicals are presented. Options for mill scale environmentally-friendly pulp bleaching are included.

Coagulation of wood extractives in chemical pulp bleaching filtrate by cationic polyelectrolytes

The purpose of this research was to investigate the effectiveness of short-chain cationic polyelectrolytes of different molecular weights and charge densities in reducing turbidity and selectively removing toxic wood extractives from chemical birch pulp filtrate. The effects of chemical type, dosage and temperature were of interest. An effective performance was achieved with a copolymer of acrylamide and methacrylate of medium molecular weight and medium charge density at 72 °C and pH 5–6. The dosage range optimum for reducing the turbidity was 102–142 mg/L. Up to 92% of the wood extractives was selectively removed.

Ozonation of conventional kraft and SuperBatch residual lignins in methanol/water and water

Abstract The chemistry of ozone bleaching of chemical pulps was explored by ozonating residual lignins isolated from conventional kraft and SuperBatch pulps in methanol/water medium to detect possible differences in the reactivity of the two types of lignin. SuperBatch lignin was ozonated also in water to study the effect of ozonation medium on the lignin reactivity. The residual lignins were found to display similar reactivities in methanol/water, implying that ozonation should result in equal delignification rates for both conventional kraft and SuperBatch pulps unless the rates of reagent diffusion in the pulps are different. The lignins were partly oxidized to volatile and nonvolatile low-molecular weight oxidation products by the so-called “peeling mechanism”, according to which oxidation products go into solution and insoluble reaction products resemble the starting lignins. The reaction products obtained upon ozonation of SuperBatch lignin in neutral water resembled those formed in methanol/water, but their yield was much lower. This is probably due to the better solubility of lignin in methanol/water than in water and/or higher degradability of lignin by ozone than by radicals formed as ozone decomposition products.

Biological Bleaching of Chemical Pulps

ABSTRACT: Use of biotechnology in pulp bleaching has attracted considerable attention and achieved interesting results in recent years. Enzymes of the hemicellulolytic type, particularly xylan-attacking enzymes, xylanases are now used commercially in the mills for pulp treatment and subsequent incorporation into bleach sequences. The aims of the enzymatic treatment depend on the actual mill conditions and may be related to environmental demands, reduction of chemical costs or maintenance or even improvement of product quality. The use of oxidative enzymes from white-rot fungi, that can directly attack lignin, is a second-generation approach, which could produce larger chemical savings than xylanase but has not yet been developed to the full scale. It is being studied in several laboratories in Canada, Japan, the U.S.A. and Europe. Certain white-rot fungi can delignify kraft pulps increasing their brightness and their responsiveness to brightening with chemicals. The fungal treatments are too slow but the enzyme manganese peroxidase and laccase can also delignify pulps and enzymatic processes are likely to be easier to optimize and apply than the fungal treatments. Development work on laccase and manganese peroxidase continues. This article presents an overview of developments in the application of hemicellulase enzymes, lignin-oxidizing enzymes and white-rot fungi in bleaching of chemical pulps. The basic enzymology involved and the present knowledge of the mechanisms of the action of enzymes as well as the practical results and advantages obtained on the laboratory and industrial scale are discussed.

Optimization and Control of Hydrogen Peroxide Bleaching

Hydrogen peroxide bleaching is nowadays used not only for mechanical pulp but also for chemical pulp in delignifying OP-or PO-stages (the notification depends of if oxygen or peroxide is the dominating one).The method discussed in this paper is primarily developed for bleaching of mechanical pulp, but is in principal applicable also for bleaching of chemical pulp. If the bleaching is carefully optimized with respect to total alkali, the brightness will be dependent on the peroxide consumption.The peroxide consumption is basically a function of pulp consistency and reaction time during bleaching. Mathematical models developed by Dr. Moldenius for hydrogen peroxide can be used for both one-and two-stage bleach plants.In both cases a system with recycling of residual peroxide is used and compensating the flow of fresh peroxide to obtain a constant feed into the bleach tower.This requires an accurate measurement of the total amount of residual peroxide going back to the input of the bleaching tower. For this purpose, the analyzer RPA-5000 is used. The function of this analyzer will be presented in the beginning of this paper.

Quantification of Muconic Acid-Type Structures in High Molecular Weight Material from Bleach Plant Effluents

Summary Effluents from the chlorine dioxide bleaching of chemical pulps were characterized with respect to high and low molecular weight material, COD, TOC, elemental composition, and carboxylic acid and methyl ester content. A technique was developed for the quantitative estimation of muconic acid-type structures as moities of the high molecular weight material. The method is based on the use of quantitative 13C NMR and the pH-dependent cyclization of muconic acids in aqueous solution. 20–40 % of the carboxylic acids in the high molecular weight material isolated from bleach plant effluents from three softwood kraft mills were found to exhibit muconic acid-type behaviour.

Prevention of Fouling to Cation Exchange Membrane in Electrolyzer for On-site Production of Alkaline Hydrogen Peroxide.

Hydrogen peroxide is one of the most important chemicals used in deinking of waste paper and bleaching of mechanical pulp. Environmental restrictions on the use of chlorine-based bleaching techniques are forcing the use of hydrogen peroxide in chemical-pulp bleaching. Therefore a demand for the on-site production of hydrogen peroxide is increasing in the pulp and paper industry. The trickle-bed electrolyzer was developed with a cation-exchange membrane, which could prevent the loss of current efficiency (back-migration of the perhydroxy ion through separator), for the electrochemical production of the hydrogen peroxide. And in order to verify the long-term performance of the electrolyzer, the operational condition of the electrolyzer was investigated. Although the cation-exchange membrane was fouled during continuous operation, we found that an addition of an ethylenediaminetetra-acetic acid (EDTA) salt into the anolyte improved the current efficiency and prevent fouling of the ion-exchange membrane. Anolyte recycling was carried out for a practical usage of the anolyte. The cation-exchange membrane was fouled with metallic compounds from make-up NaOH solution when anolyte was recycled 47 times. These metallic compounds were effectively eliminated from the make-up solution by filtration treatment after mixing with magnesium sulfate. It was also clarified that 10% anolyte purge was effective to keep the lower metal concentration from the critical level. The possibility of the trickle-bed electrolyzer is discussed for the on-site electrochemical production of the hydrogen peroxide.

Mechanisms of AOX Removal During Biological Treatment

It is well known that a portion of the AOX generated during the bleaching of chemical pulps will degrade to inorganic chloride when alkali and heat are added to the waste stream. As reported by others, this hydrolysis can potentially occur in the mill sewer system as well as within the wastewater treatment process. The present study was undertaken to examine the role which alkaline hydrolysis can be expected to play in the removal of AOX during biological treatment. The results of these experiments indicate that, for the filtrates tested, (a) biodegradation can account for 75 percent of the total AOX removal across secondary treatment, where pretreatment of bleach plant filtrates is not provided, and (b) approximately forty percent of the AOX destroyed under harsh thermo-alkaline conditions is biodegradable AOX and approximately one-quarter of the AOX destroyed would be removed by alkaline hydrolysis in the secondary treatment process itself. This suggests that biological and thermoalkaline treatment processes target many of the same AOX components.

Improving Peroxide Bleaching of Chemical Pulps by Stabilizing Manganese

Based on the earlier results that unlike Mn(III), Mn(II) is not catalytically active towards manganese induced peroxide decomposition under alkaline conditions, we studied to improve peroxide bleaching of an oxygen delignified pulp by reducing the manganese in the pulp fibers from its high oxidation state and then stabilizing the reduced manganese with additives such as DTPA. It was shown that pulp fibers under an acidic condition are effective reducing agents and so is sodium hydrosulfite. With such a strategy it was found that the peroxide decomposition catalyzed by the residual manganese present in pulp fibers can be minimized and, consequently, the peroxide bleaching performance can be improved.

Effect of reaction medium on ozone mass transfer and applications to pulp bleaching

Over the last few years, ozone has been considered as an effective reagent for the bleaching of chemical pulp and an alternative to conventional chlorinated compounds which induce environmental pollution. The purpose of this work was to investigate the optimal conditions of the ozonation process with the aim of avoiding the breakdown of carbohydrates present in the pulp. In order to limit the depolymerization of cellulose, which involves a decrease in the pulp strength, various organic compounds called “protectors” were added to the reaction medium. The results reported in this paper show the important role of most additives and their influence on the hydrodynamic phenomena taking place in the ozonation process. Indeed, these products improve the mass transfer and, more particularly, the diffusion of ozone in the pulp. Consequently, their presence enhances the performance of the ozonation stage.

Mechanisms of AOX removal during biological treatment

It is well known that a portion of the AOX generated during the bleaching of chemical pulps will degrade to inorganic chloride when alkali and heat are added to the waste stream. As reported by others, this hydrolysis can potentially occur in the mill sewer system as well as within the wastewater treatment process. The present study was undertaken to examine the role which alkaline hydrolysis can be expected to play in the removal of AOX during biological treatment. The results of these experiments indicate that, for the filtrates tested, (a) biodegradation can account for 75 percent of the total AOX removal across secondary treatment, where pretreatment of bleach plant filtrates is not provided, and (b) approximately forty percent of the AOX destroyed under harsh thermo-alkaline conditions is biodegradable AOX and approximately one-quarter of the AOX destroyed would be removed by alkaline hydrolysis in the secondary treatment process itself. This suggests that biological and thermoalkaline treatment processes target many of the same AOX components.

Photochemical bleaching of chemical pulps catalyzed by titanium dioxide

A two-stage process for photochemical bleaching of cellulosic pulps is presented. The first, based on the generation of oxygen active species by the photocatalytic action of TiO 2, and the second on the photochemical decomposition of hydrogen peroxide. Both stages are carried out under alkaline pH and at 85°C in aqueous suspension at a consistency of 5%. The experiments were performed on kraft ( Eucalyptus grandis, Pinus pinaster and Picea mariana), acesolv ( E. grandis) and peroxyformic acid ( E. grandis) pulps. The presence of TiO 2 as photocatalyst showed several advantages, such as reduction of reaction time, preservation of the pulp viscosity, increase of the selectivity during the photobleaching and decrease of the consumption of the bleaching chemicals. UV/Vis and FTIR spectroscopies indicate a decrease of the coniferaldehyde structures during the TiO 2 photocatalyzed stage, whereas quinones entities were found to remain in the residual lignin even after the final hydrogen peroxide stage. TiO 2-photocatalyzed sequence was found to degrade more efficiently the chromophores, especially carbonyl groups, than the sequence carried out in its absence.

過酸化水素漂白 欧米の紙パルプ産業における過酸化水素利用の動向

The demand for hydrogen peroxide in the Pulp and Paper industry in Europe and North America started with the brightening of Mechanical Pulp and the bleaching of Recycled Pulp as it is presently the common practice in Japan.The consumption for bleaching of Chemical Pulp has been increased since 10 years ago, in order to the trend to go to the environment friendly process. The European bleached kraft pulp mills avoided using Sodium Hypochlorite. One possible solution is the use of H2O2 and also possible to operate a partial substitution by ClO2.The trend for TCF and ECF bleaching sequence is very in favor of the utilization of H2O2 as pressurized hydrogen peroxide bleaching sequence for kraft pulp beyond the Eop, P sequences. In Western Europe and in North America, the H2O2 use in chemical pulp mills is on average 82% and 67%.